Fresh allegations about a regime of torture and humiliation inflicted on detainees by their American captors at Guantánamo Bay have been made by a Briton still held there, according to Foreign Office documents seen by the Guardian.

The claims by Martin Mubanga, from London, are the latest to surface from the prison where the US holds 550 Muslim men it claims are terrorists in conditions that have sparked worldwide condemnation.

Mr Mubanga, 31, alleges that only months ago he was kept shackled for so long that he wet himself, and then was forced to clean up his own urine. He claims to have been threatened, that an interrogator stood on his hair, and that he was subjected to extremes of temperature rising to 36C (97F). He was kept chained to the floor by his feet for an hour during a welfare visit from a British government official.

Mr Mubanga is one of four Britons still in Guantánamo, held without charge or trial. His allegations follow revelations that FBI agents recently raised concerns about ill treatment they witnessed of prisoners.

The claim of fresh abuses against Britons came as Lord Falconer, the lord chancellor, ventured further than any senior government figure so far in attacking the US policy at Guantánamo Bay. "At the heart of our culture is a commitment to the rule of law and human rights," the lord chancellor said in a speech to the Institute of Public Policy Research, a leftwing thinktank. "We could never countenance individuals being put beyond the law as has happened at Guantánamo Bay."

That period of being beyond the rule of law had now ended, he added, as a result of a supreme court decision allowing the detainees to challenge their incarceration in court.

But Louise Christian, solicitor for two British detainees, including Mr Mubanga, dismissed Lord Falconer's comments: "There is no difference between what we are doing here at Belmarsh and Guantánamo Bay. People at both are becoming mentally ill."

Other allegations of mistreatment at Guantánamo were made in October in a letter from another of the British detainees, Moazzam Begg. The five Britons who were released from Guantánamo in March have also alleged torture.

The allegations about Mr Mubanga's treatment are contained in a letter from a Foreign Office official to the prisoner's family. Mr Mubanga, a former motor cycle courier, made his allegations in June during a welfare visit.

The letter reads: "Martin told the official ... he had been interrogated, shackled and not allowed to go to the toilet.

"He said he had wet himself and had been forced to clean up the mess himself. Martin said that in another incident in June, he had been put in a room with the temperature at 97F ... he knew the temperature because he had seen the dial."

The letter continues: "Martin said that there had been a struggle and he had had his hair stood on by the interrogator."

Mr Mubanga was arrested in Zambia and has been held as a terrorist for more than two years without access to a lawyer. During a visit by a Foreign Office official on October 3, a record of which the Guardian has seen, Mr Mubanga was kept trussed up for the entire 60 minutes. The official noted: "Martin's feet were shackled to the ring in the floor." Mr Mubanga also said his weight had plunged in captivity from 84kg to 75kg (13st 3lb to 11st 11lb), and that he got tired and dizzy and did not get enough food.

He is allowed 30 minutes of exercise every second day.

Ms Christian said the allegations showed the failure of government efforts to secure the Britons' release. "For the British government to sit on its hands while its closest ally, the US government, is torturing our citizens is unforgivable," she said.

According to the Foreign Office letter, the US claims Mr Mubanga attacked his interrogator, despite the fact he almost certainly would have been shackled: "The US authorities said that their records show that Martin grabbed the interrogator's hand and applied a pressure point hold. The interrogator stated that he would call the military police and Martin let go without further incident."

A Foreign Office spokeswoman said: "We have raised with the US authorities the allegations of maltreatment raised with us by Mr Mubanga during a welfare visit. The US authorities have investigated them and their response is that they are without merit.

"At this stage we would not propose to pursue this further."

The Foreign Office had originally refused to give Mr Mubanga's family details of his claims of ill-treatment, blaming the data protection act.